<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        body {
            background: orange;
        }
    </style>
</head>
<body>
    <h1>child</h1>
    <iframe src="./child-child.html" frameborder="0" width='50px' height='50px'></iframe>
    <script>
        var b = 10;
        // console.log(window.parent.a);
        // 返回父窗口的window
    </script>
</body>
</html>